1. Field of the Invention
The present invention relates generally to a method of monitoring a CVD liquid source used for forming a thin film having a high dielectric constant, and more particularly to a method of monitoring a CVD liquid source for forming a thin film having a high dielectric constant, in order to detect presence of abnormal concentration and deterioration of the CVD liquid source.
The present invention relates further to an improved monitoring apparatus for a CVD liquid source allowing the detection of presence of abnormal concentration and deterioration of the CVD liquid source.
2. Description of the Background Art
Recently the degree of integration of semiconductor memories and devices has been rapidly increasing. In a dynamic random access memory (DRAM), for example, a device capacity (bit number) has increased at an astonishing rate of four-fold in three years. An object to increase the integration of devices is the reduction of power consumption, cost and so on. Regardless of the higher degree of integration, however, a capacitor, a component of a DRAM, must maintain a certain capacity. For this purpose, it is necessary to reduce the thickness of a film used as a capacitor, which is unachievable when a conventional material, that is, SiO.sub.2, is used. A larger capacity can be obtained by increasing the dielectric constant of a film by using a different material from those employed heretofore as well as by reducing the film thickness. Thus studies for utilizing the materials having a high dielectric constant for a memory device are now enthusiastically conducted.
As long as a material having a high dielectric constant is used, a leakage current and the thickness of a film must be minimized. Generally, it is desirable that the film thickness is at most 0.5 nm in SiO.sub.2 equivalent and that a leakage current density at the application of 1V is at most 2.times.10.sup.-7 A/cm.sup.2.
The conventional method has a drawback when it is employed for forming a thin film over a capacitor electrode of a DRAM having a step such as shown in FIG. 5.
Referring to FIG. 5, a DRAM includes a semiconductor substrate 1, on which an interlayer insulation film 2 is formed. A storage node 3 is formed on interlayer insulation film 2. A contact hole 4 is formed in interlayer insulation film 2 in order to connect storage node 3 and semiconductor substrate 1, which are connected by a conductive material filled in contact hole 4. A capacitor insulation film 5 is formed on interlayer insulation film 2 such that it covers storage node 3. A cell plate 6 coats storage node 3, and capacitor insulation film 5 is posed therebetween.
For forming a thin film over a capacitor electrode of a DRAM having a step, as shown in FIG. 5, a CVD method is regarded as the most advantageous, which attains a good coverage to a body with a complicated configuration. Sources used for the method, however, do not have a stable and favorable vaporization characteristic. The heat-induced vaporization characteristic of dipivaloylmethane (DPM) compound of .beta.-diketon type, which is commonly used as a CVD material, is not preferable.
Against the above-described background, the present inventors found a novel CVD source having a dramatically improved vaporization characteristic and being produced by dissolving a conventional solid material in an organic solvent called tetrahydrofuran (THF). When a conventional CVD apparatus for a liquid source, forming a SiO.sub.2 film is used, however, a good dielectric film is not always acquired from this novel source. Thus a novel CVD apparatus for a liquid source was proposed which allows a sufficient vaporization and constant feeding of a liquid source to a reaction chamber (Japanese Patent Laying-Open No. 6-310444). In addition the inventors have found that a coverage can be surprisingly improved by changing a Ti material from TTIP (Ti(O-i-Pr).sub.4) commonly used as a Ti material to TiO(DPM).sub.2 of DPM type. The inventors have further found that, compared with a single-layer film, a two-step films deposition is effective in order to attain a good surface morphology and electric characteristics, in which a second layer of a film is deposited over a first one crystallized by annealing at the starting phase of film deposition where the film being formed is likely to have a relatively amorphous feature. (Japanese Patent Laying-Open No. 7-268634).
Additionally the installation of a FT-IR, for example, to the CVD apparatus for a liquid source is proposed in order to allow an optical in-situ monitoring of film deposition. A lower electrode structure suitable for the deposition of a BST film (which includes Ba, Sr and Ti) by the liquid source CVD method were also found (Japanese Patent Laying-Open Nos. 8-176826 and 8-186103).
It was found, however, that the dielectric films formed by employing the liquid source CVD apparatus do not always have a stable and favorable film quality (including electric characteristics).
FIG. 6 is a schematic diagram of an example of a liquid source CVD apparatus as the one disclosed in Japanese Patent Laying-Open No. 9-219497.
Referring to FIG. 6, the liquid source CVD apparatus includes a reaction chamber 22 which houses a substrate 24 to be heated by a substrate heater 23. A reactive gas supplying tube 20 is connected to reaction chamber 22. A heater 21 is also attached to reaction chamber 22. Reaction chamber 22 and a vaporizer 12 are mutually connected by a transport tube 17. Transport tube 17 is provided with a heater 18 for heating transport tube 17.
Vaporizer 12 is provided with a source gas supplying outlet 19 and a heater 16 for heating vaporizer. The liquid source CVD apparatus includes a liquid source vessel 13 having a pressure tube 25. A liquid source feeder 14 is attached to liquid source vessel 13. The liquid source CVD apparatus also includes a solvent vessel 28.
Pressure tube 25 and a solvent feeder 29 are attached to solvent vessel 28. The liquid source CVD apparatus is provided with a carrier-gas introducing tube 26 having a carrier-gas volume controller 27. Liquid source, solvent and carrier-gas are introduced to vaporizer 12 through an vaporization nozzle 15.
The operation of the liquid source CVD apparatus shown in FIG. 6 will then be described. Carrier-gas flows with its flow rate controlled by carrier-gas volume controller 27. The liquid source in liquid source vessel 13 is introduced thereto while being pressurized by pressure tube 25 and controlled by liquid source feeder 14 and, sprayed into vaporizer 12 via vaporization nozzle 15. Source gas vaporized in vaporizer 12 is supplied from source gas supplying outlet 19 through source gas transport tube 17 to reaction chamber 22. While source gas is in source gas transport tube 17, it is heated by heater 18. In reaction chamber 22 source gas is reacted with an oxidizing agent (O.sub.2) whereby a film with a high dielectric constant (a BST film, for example) is formed on substrate 24. Three liquid source supplying systems of Ba, Sr and Ti each including elements 13 and 14 are provided, although not shown in FIG. 6. Liquid source from these three systems are together supplied to vaporizer 12. Reaction chamber is filled with an O.sub.2 atmosphere and pressure is set in the range from 1 to 15 Torr. A substrate temperature in reaction chamber 22 is set in a relatively low range of 400 to 600.degree. C. by the heater because a favorable coverage is obtained at a lower temperature. The flow rate of sources and the time for film deposition are controlled as to produce a film with the thickness of 300 .ANG. and the BST film composition ratio of (Ba+Sr)/Ti=1.0 at the film deposition rate of about 30 .ANG./min. An upper electrode Pt or Ru with a diameter of approximately 1 mm is formed by sputtering on a BST film which is formed on a lower electrode Pt or Ru, whereby electric characteristics, that is, a leakage current, equivalent SiO.sub.2 film thickness and so on, are measured.
The foregoing is the description of a conventional fabrication procedure of a thin film having a high dielectric constant.
When a CVD method is employed to form a BST film using a liquid source produced by dissolving an organometallic compound of DPM type in an organic solvent, however, the quality of the BST film is subjected to an abrupt change at the time of the replacement of a vessel of the CVD liquid source. Even when the same CVD liquid source is used, the quality of BST films fluctuates because of the deterioration of the liquid source. These are some of the reasons a film having a stable and favorable quality (including electric characteristics) cannot be obtained.